In the light of the present invention following discussions on the related art are given.
The practical application of optical communication systems in recent years has been accompanied by demand for highly sophisticated communications systems of greater capacity and increased functionality. This has made it necessary to add on new functions for generating optical signals at higher speeds, for wavelength multiplexing in the same optical transmission path and for the changeover and switching of optical transmission paths. In particular, the practical application of optical fiber amplifiers has heightened expectations in regard to wavelength division multiplexing (WDM)-technology.
In order to realize WDM transmission, reducing the wavelength dependence of optical fiber amplifier gain is essential. One method of achieving this is to optimize the gain wavelength dependence of an erbium-doped fiber (EDF). To accomplish this, however, it is required that the gain of the EDF be controlled so as to be held constant.
When the gain of an EDF is held constant, however, a fluctuation in the level of the input signal to the optical fiber amplifier appears in the output stage. If multistage transmission is carried out, level fluctuation in the receiver stage becomes large in magnitude, making application of this technique to an actual apparatus difficult.
In an effort to solve this problem, an arrangement, e.g., as shown in FIG. 4 has been proposed. Specifically, the fluctuation in the output level of an optical fiber amplifier is compensated for by an electrically controlled variable optical attentuator 404 in which feedback control is possible. Incident light is multiplexed onto excitation light from an excitation laser diode (LD) 401 by a WDM coupler 402, and the resulting signal is amplified by an erbium-doped optical fiber 403. The exiting light is variably attenuated in the variable optical attenuator 404 by a control signal voltage from an optical sensor 405 that monitors the emitted light, and the attenuated light is output via a tap coupler 406.
The structure and functions of the optical waveguide electrically controlled variable attenuator according to the prior art will now be described.
FIG. 5 illustrates an example of an optical waveguide electrically controlled variable attenuator using an optical switch of directional coupler type according to the prior art. The arrangement depicted in FIG. 5 is described in "Rendering an LiNbO.sub.3 Optical Waveguide Switch Independent of Polarization" by Mitsukazu Kondo, Yasuhisa Tanizawa, Masaaki Iwasaki, Yoshinori Ota, Tsutomu Aoyama and Roh Ishikawa, Society of Electronics Information and Communications, Semiconductor Materials Section, National Conference, published March, 1987, pp. 2.about.140.
As shown in FIG. 5, an optical waveguide 7 is fabricated in the shape illustrated in the surface of a substrate 12a comprising a lithium niobate (LiNbO.sub.3) Z plate (Z-cut crystal). A portion having an index of refraction slightly higher than that of the substrate 12a is an optical waveguide 2. The optical waveguide 7 is formed by Titanium (Ti) thermal diffusion method into the substrate. A buffer layer 13a consisting of silicon dioxide (SiO.sub.2) is provided on the optical waveguide 7, and control electrodes 8a, 8b are provided on the buffer layer 13a.
With this optical waveguide electrically controlled variable attenuator according to the prior art, signal light 3a from an erbium-doped fiber impinges upon an input port 2a. The component of the electric field of the entrant light that is parallel to the substrate is TE (Transverse Electric) mode 4, and the component of the electric field of the entrant light that is orthogonal to the substrate is TM (Transverse Magnetic ) mode 5.
In a case where voltage is not being applied upon the control electrodes 8a, 8b, guided light 6a that has entered from the input port 2a is acted upon in a directional coupler 11a in such a manner that optical power is transferred from optical waveguide 7b to the adjacent optical waveguide 7d, travels through optical waveguide 7c on the output side and exits to an output fiber 1b from an output port 2b.
In this optical waveguide electrically controlled variable attenuator according to the prior art, the directional coupler switch 11a is designed to have such a switch length (the length of the switch decided by optical waveguides 7b, 7d, referred to also as "complete coupling length") that the optical power will be transferred completely between the optical waveguides 7a, 7d. When voltage V (9a) is applied upon the control electrodes 8a, 8b in the directional coupler switch 11a, the index of refraction of the optical waveguides 7b, 7d immediately underlying the electrodes changes owing to electro-optic effects, as a result of which the intensity of the output light 3b changes.
Intensity P of the output light 3b in relation to the control voltage V(9a) is represented by Equation (1) below. For example, see "Optical Integrated Circuits", Nishihara et al., published by Ohm-sha, 1985. ##EQU1##
FIG. 6 illustrates an example of the attenuation characteristic of the output light 3b plotted against the control voltage V(9a). More specifically, the control voltage V(9a) is plotted along the horizontal axis and the amount of attenuation (expressed in decibels) is plotted along the vertical axis. Here the intensity of light is normalized by the optical waveguide 6a, and the applied voltage is normalized by a voltage (referred to also as "switching voltage") VTM which attenuates the TM mode light to the maximum extent. Accordingly, by controlling the control voltage V(9a), the attenuator makes it possible to vary the amount of attenuation of the input light 3a at will.
FIG. 7 illustrates another example of the construction of an optical waveguide electrically controlled variable attenuator using an optical switch of directional coupler type according to the prior art.
As shown in FIG. 7, the conventional attenuator is such that the optical waveguide 7 is fabricated in the shape illustrated in the surface of the substrate 12a made up of a lithium niobate (LiNbO.sub.3) X-plate(X-cut crystal). A portion having an index of refraction slightly higher than that of the substrate 12a is the optical waveguide 2. The optical waveguide 7 is formed by Titanium (Ti) thermal diffusion method into the substrate. A buffer layer 13a consisting of silicon dioxide (SiO.sub.2) is provided on the optical waveguide 7, and control electrodes 8a, 8b are provided on the buffer layer 13a.
With this electrically controlled variable optical attenuator of optical waveguide type according to the prior art, the signal light 3a from an erbium-doped fiber impinges upon the input port 2a. The component of the electric field of the entrant light that is parallel to the substrate is TE (Transverse Electric) mode 4, and the component of the electric field of the entrant light that is orthogonal to the substrate is TM (Transverse Magnetic) mode 5.
In a case where voltage is not being applied upon the control electrodes 8a, 8b, guided light 6a that has entered from the input port 2a is acted upon in the directional coupler 11a in such a manner that optical power is transferred from optical waveguide 7b to the adjacent optical waveguide 7d, travels through optical waveguide 7c on the output side and exits to an output fiber 1b from an output port 2b.
In this optical waveguide electrically controlled variable attenuator according to the prior art, the directional coupler switch 11a is designed to have such a switch length (the length of the switch decided by optical waveguides 7b, 7c, referred to also as "complete coupling length") that the optical power will be transferred completely between the optical waveguides 7b, 7c. When voltage V (9a) is applied upon the control electrodes 8a, 8b in the directional coupler switch 11a, the index of refraction of the optical waveguides 7b, 7c immediately underlying the electrodes changes owing to electro-optic effects, as a result of which the intensity of the output light 3b changes.
FIG. 8 illustrates an example of the attenuation characteristic of a TE mode 4b and a TM mode 5b plotted against the control voltage 9a. Here the intensity of light is normalized by the optical waveguide 6a, and the applied voltage is normalized by a voltage (referred to also as "switching voltage") VTM which attenuates the TM mode to the maximum extent. Accordingly, by controlling the control voltage V(9a), the attenuator makes it possible to vary the amount of attenuation of the input light 3a at will.